Hmmm... that's a toughy, I heard that you can know by its color changing.
But if overdoing it, it can be like a dull color, for example take Orange soda for the example, thats overdoing it, its too dull, now like flavored orange water, its just right, I seen it when i was in chemistry class.
NOTE: I hope this helps really much, if it doesn't, Then I'm sorry.